> This PR the implementation of "JEP 376: ZGC: Concurrent Thread-Stack > Processing" (cf. > https://openjdk.java.net/jeps/376). > Basically, this patch modifies the epilog safepoint when returning from a > frame (supporting interpreter frames, c1, c2, > and native wrapper frames), to compare the stack pointer against a > thread-local value. This turns return polls into > more of a swiss army knife that can be used to poll for safepoints, > handshakes, but also returns into not yet safe to > expose frames, denoted by a "stack watermark". ZGC will leave frames (and > other thread oops) in a state of a mess in > the GC checkpoint safepoints, rather than processing all threads and their > stacks. Processing is initialized > automagically when threads wake up for a safepoint, or get poked by a > handshake or safepoint. Said initialization > processes a few (3) frames and other thread oops. The rest - the bulk of the > frame processing, is deferred until it is > actually needed. It is needed when a frame is exposed to either 1) execution > (returns or unwinding due to exception > handling), or 2) stack walker APIs. A hook is then run to go and finish the > lazy processing of frames. Mutator and GC > threads can compete for processing. The processing is therefore performed > under a per-thread lock. Note that disarming > of the poll word (that the returns are comparing against) is only performed > by the thread itself. So sliding the > watermark up will require one runtime call for a thread to note that nothing > needs to be done, and then update the poll > word accordingly. Downgrading the poll word concurrently by other threads was > simply not worth the complexity it > brought (and is only possible on TSO machines). So left that one out.
Erik Österlund has updated the pull request incrementally with one additional commit since the last revision: Review: StefanK CR 3 ------------- Changes: - all: https://git.openjdk.java.net/jdk/pull/296/files - new: https://git.openjdk.java.net/jdk/pull/296/files/3db0dbcb..078a207e Webrevs: - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=296&range=05 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=296&range=04-05 Stats: 12 lines in 3 files changed: 9 ins; 0 del; 3 mod Patch: https://git.openjdk.java.net/jdk/pull/296.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/296/head:pull/296 PR: https://git.openjdk.java.net/jdk/pull/296
